Since about 1920, chemically precipitated calcium carbonate has been used as a pigment or filler in the paper industry. Various chemical routes have been followed to precipitate the calcium carbonate, but the most frequently used methods are based on the double decomposition of sodium carbonate with either calcium hydroxide or calcium chloride, or on the carbonation with carbon dioxide gas of an aqueous suspension of calcium hydroxide ("milk of lime"). Double decomposition processes generally employ by-products of other chemical processes and therefore tend to yield calcium carbonate products which contain unwanted salts. The process based on the carbonation of milk of lime is performed in three stages; firstly, the calcination of raw limestone to produce calcium oxide or "quicklime"; secondly, the "slaking" of the quicklime with water to produce an aqueous suspension of calcium hydroxide; and finally, the carbonation of the calcium hydroxide with a gas comprising carbon dioxide.
In order to prepare a precipitated calcium carbonate for the paper industry, a process based upon the carbonation of milk of lime is preferred because there is no serious problem of contamination of the product with unwanted salts, and each of the three stages in the production process can be controlled to adjust the properties of the final product.
Calcium carbonate can be precipitated from aqueous solution in three different crystal forms: the vaterite form which is thermodynamically unstable, the calcite form which is the most stable and the most abundant in nature, and the aragonite form which is metastable under normal ambient conditions of temperature and pressure, but converts to calcite at elevated temperature. The aragonite form crystallises as long, thin needles having a length:diameter ratio of about 10:1, but the calcite form exists in several different shapes of which the most commonly found are the rhombohedral shape in which the length and the diameter of the crystals are approximately equal, and the crystals may be either aggregated or unaggregated; and the scalenohedral shape in which the crystals are like double, two-pointed pyramids having a length:width ratio of about 4:1, and which are generally unaggregated. All these forms of calcium carbonate can be prepared by carbonation of milk of lime by suitable variation of the process conditions.
A particularly desirable type of pigment for the paper industry is known as a "bulking pigment". The opacity and brightness of a paper sheet filled or coated with a mineral material depend on the ability of the sheet to scatter light. If the pigment consists of fine particles which are separated by small spaces or voids, the scattering effect is generally enhanced, and is found to be at an optimum when the width of the spaces or voids is about half the wavelength of visible light, or about 0.25 microns. Bulking pigments, or pigments consisting of fine particles separated by spaces or voids of about the optimum size, are desirable in the paper industry on account of their ability to scatter visible light, but if the pigment consists of discrete fine particles, the retention of these particles in a matrix of cellulosic papermaking fibres is poor. To obtain good retention, the fine particles must be aggregated together to form clusters of larger size.
High light scattering pigments currently available to the paper industry include titanium dioxide, which is very effective but also expensive, and fine kaolin particles which have been aggregated either thermally or chemically. Pigments derived from kaolin are also effective in scattering light, but are again expensive. Of the various forms of calcium carbonate, the aragonite form is effective as a high light scattering pigment but the process conditions necessary for its production are stringent and difficult to control. The rhombohedral form has crystals which are generally unaggregated and which pack together too closely and do not leave between them voids or spaces of the appropriate size. The scalenohedral form may be produced relatively inexpensively and the process conditions may be readily controlled to give aggregates of fine crystals separated by spaces of substantially the optimum size for light scattering, and is therefore the preferred form of calcium carbonate for use as a bulking pigment in the paper industry.
U.S. Pat. No. 2,081,112 (N. Statham & T.G. Leek) describes a process for producing precipitated calcium carbonate by carbonating milk of lime. It is recognised that the more violent the agitation in the gas absorber, the finer will be the product, and the aim is to create a fine mist of calcium hydroxide slurry in the presence of the carbon dioxide-containing gas. The temperature in the gas absorber is maintained at 50.degree.-60.degree. C., preferably around 55.degree. C.
U.S. Pat. No. 2,964,382 (G.E. Hall, Jnr) concerns the production of precipitated calcium carbonate by various chemical routes in which calcium ions are contacted with carbonate ions in a precipitation zone, including the carbonation of milk of lime. High shear, intense turbulence is provided in the precipitation zone by means of an impeller rotating at a peripheral speed of at least 1160 feet per minute (589 cm. per second).
U.S. Pat. No. 3,320,026 (W.F. Waldeck) describes the production of different forms of calcium carbonate including the scalenohedral form. The calcium hydroxide is relatively coarse and contains at least 50% by weight of particles larger than 10 microns. The temperature in the gas absorber is maintained at less than 20.degree. C.
U.S. Pat. No. 4,018,877 (R.D.A. Woods) describes a carbonation process in which there is added to the suspension in the gas absorber, after the calcium carbonate primary nucleation stage and before completion of the carbonation step, a complexing agent for calcium ions, such as ethylenediamine tetraacetic acid (EDTA), aminotriacetic acid, aminodiacetic acid or a hydroxy polycarboxylic acid.
U.S. Pat. No. 4,157,379 (J. Arika et al) describes the production of a chain-structured precipitated calcium carbonate by the carbonation of calcium hydroxide suspended in water in the presence of a chelating agent and a water-soluble metal salt.
U.S. Pat. No. 4,367,207 (D.B. Vanderheiden) describes a process in which carbon dioxide-containing gas is introduced into an aqueous calcium hydroxide slurry containing an anionic organpolyphosphonate electrolyte to give a finely divided precipitated calcium carbonate.